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absolute brightness (magnitude) - See luminosity
absolute zero - A temperature of -273°C (or 0° K), where all molecular motion stops.
absorption spectrum - Dark lines superimposed on a continuous spectrum.
accelerate - To change velocity; to speed up, slow down, or change direction.
accretion - Gradual accumulation of mass, as by a planet forming by the building up of colliding particles in the solar nebula or gas falling into a black hole.
accretion disk  - A disk of matter spiraling in toward a massive object; the disk shape is the result of conservation of angular momentum.
active galactic nucleus  - A galaxy is said to have an active nucleus if unusually violent events are taking place in its center, emit-ting large quantities of electromagnetic radiation. Seyfert galaxies and quasars are examples of galaxies with active nuclei. 
active region  - Areas on the Sun where magnetic fields are concentrated; sunspots, prominences, and flares all tend to occur in active regions. 
albedo - The fraction of incident sunlight that a planet or minor planet reflects.
alpha particle  - The nucleus of a helium atom, consisting of two protons and two neutrons.
amplitude - The range in variability, as in the light from a variable star.
angular diameter  - Angle subtended by the diameter of an object. 
angular momentum  - A measure of the momentum associated with motion about an axis or fixed point. 
antapex (solar)  - Direction away from which the Sun is moving with respect to the local standard of rest.
Antarctic Circle  - Parallel of latitude 66°30' S; at this latitude the noon altitude of the Sun is O° on the date of the summer solstice.
antimatter - Matter consisting of antiparticles: antiprotons (protons with negative rather than positive charge), positrons (positively charged electrons), and antineutrons. 
aperture  - The diameter of an opening, or of the primary lens or mirror of a telescope. 
apex (solar)  - The direction toward which the Sun is moving with respect to the local standard of rest. 
aphelion - Point in its orbit where a planet (or other body) is farthest from the Sun. 
apparent brightness  - A measure of the observed light received from a star or other object at the Earth; i.e., how bright an object appears in the sky, as contrasted with its luminosity. 
Arctic Circle  - Parallel of latitude 66°30' N; at this latitude the noon altitude of the Sun is 0° on the date of the winter solstice.
array (interferometer)  - A group of several telescopes that is used to make observations at high angular resolution. 
association - A loose group of young stars whose spectral types, motions, or positions in the sky indicate that they have probably had a common origin. 
asterism - An especially noticeable star pattern in the sky, such as the Big Dipper. 
asteroid - An object orbiting the Sun that is smaller than a major planet, but that shows no evidence of an atmosphere or of other types of activity associated with comets. Also called a minor planet. 
asteroid belt  - The region of the solar system between the orbits of Mars and Jupiter in which most asteroids are located. The main belt, where the orbits are generally the most stable, extends from 2.2 to 3.3 AU from the Sun.
astrology - The pseudoscience that deals with the supposed influences on human destiny of the configurations and locations in the sky of the Sun, Moon, and planets; a primitive belief system that had its origin in ancient Babylonia. 
astronomical unit (AU)  - Originally meant to be the semimajor axis of the orbit of the Earth; now defined as the semimajor axis of the orbit of a hypothetical body with the mass and period that Gauss assumed for the Earth. The semimajor axis of the orbit of the Earth is 1.000000230 AU. 
atom - The smallest particle of an element that retains the properties that characterize that element. 
atomic mass unit - Chemical: one-sixteenth of the mean mass of an oxygen atom. Physical: one-twelfth of the mass of an atom of the most common isotope of carbon. The atomic mass unit is approximately the mass of a hydrogen atom, 1.67 X 10-27 kg.
atomic number  - The number of protons in each atom of a particular element.
atomic weight  - The mean mass of an atom of a particular element in atomic mass units.
aurora  - Light radiated by atoms and ions in the ionosphere, mostly in the magnetic polar regions. 
autumnal equinox  - The intersection of the ecliptic and celestial equator where the Sun crosses the equator from north to south. A time when every place on Earth has 12 hours of day-light and 12 hours of night. 
axis - An imaginary line about which a body rotates. 
azimuth - The angle along the celestial horizon, measured eastward from the north point to the intersection of the horizon with the vertical circle passing through an object.
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